It is known that when phenols, particularly alkyl-substituted phenols are oxidized under specific conditions, oxidation coupling reaction is brought about to form biphenols and diphenoquinones as reaction products. Of these products, biphenols recovered are used as antioxidants, modifiers for synthetic resins such as polyesters or polycarbonates, dyes, intermediates of pharmaceutical preparations or agricultural chemicals, or photographic chemicals.
In this connection, the oxidation coupling reaction of phenols, particularly alkyl-substituted phenols, when carried out according to the conventionally known methods, for example, such processes as taught by U.S. Pat. Nos. 3,306,874, or 3,306,875, scarcely forms the desired product biphenols, whereby mainly formed are diphenoquinones which are the oxidation products of biphenols which have been further oxidized. On that account, it was necessary to convert again the diphenoquinones formed by the above-mentioned reaction into biphenols by some measures, for example, reduction of said diphenoquinones with hydrogen in order to obtain the desired product biphenols. According to the processes disclosed in the above-mentioned U.S. Patents' specifications, there was such a problem that in addition to the diphenoquinones, biphenyl ethers, polyphenylene ethers, etc. are formed thereby. Just for information, an equation is given below to illustrate oxidation coupling reaction of 2,6-disubstituted phenol. ##STR1##
With the purpose of solving such problems referred to above, there have been proposed various processes for preparing alkyl-substituted biphenols in enhanced selectivity by controlling formation of diphenoquinones when alkyl-substituted phenols are oxidized. For example, Japanese Patent Laid-Open-to-Public Publn. No. 72131/1980 discloses a process for preparing alkyl-substituted biphenols by oxidation of alkyl-substituted phenols in the presence of alkaline catalysts, such as alkali metal hydroxides, weak acid alkali metal salts, alkaline earth metal hydroxides and weak acid alkaline earth metal salts, while strictly controlling the amount of oxygen present in the reaction system.
In the above-mentioned process, however, there is involved such a problem that because it is very difficult to control the oxygen amount so as to form no diphenoquinones, the formation of the diphenoquinones is unavoidable.
Further, Japanese Patent Publication No. 15293/1971 discloses a process for preparing alkyl-substituted biphenols, characterized by oxidizing alkyl-substituted phenols with oxygen in the presence of alkali metal hydroxides up to diphenoquinones, and incorporating the resulting reaction mixture with alkyl-substituted phenols, thereby effecting reaction between the diphenoquinones and alkyl-substituted phenols in a nitrogen atmosphere.
In this process, however, there were involved such problems that a slow rate of reaction between the diphenoquinones and alkyl-substituted phenols requires a long period of time to complete the reaction, the efficiency of the formation of desired alkyl-substituted biphenols is poor and, moreover, said process is scarcely fit for the continuous process for the preparation of the alkyl-substituted biphenols.
Still further, Japanese Patent Laid-Open-to-Public Publn. No. 140034/1983 discloses a process for preparing alkyl-substituted biphenols, characterized by oxidizing alkyl-substituted phenols in the presence of alkali catalysts to form a reaction mixture comprising alkyl-substituted biphenols and diphenoquinones, and incorporating the reaction mixture with water-immiscible organic solvents to form a two-layer system, followed by hydrogenation in the presence of hydrogenation catalysts.
In this process, however, there were involved such problems that there is a risk of explosion because of the step of reduction with hydrogen involved in this process and, moreover, a life of the hydrogenation catalyst used is short because the alkali catalyst used in the oxidation reaction acts as a catalyst poison.
The present invention is intended to solve such problems associated with the prior art as mentioned above, and its object is to provide a process for obtaining biphenols particularly dialkyl-substituted biphenols in high yields from phenols particularly alkyl-substituted phenols.